Question: What if the story goal changes halfway through the book, due to a discovery unbeknownst to the protagonist? Can a story goal change?Answer:
The story goal does not change. It sits outside the linear chronology of the story. But the protagonist/main character can pursue the wrong goal for some or most of the story.
Dramatica explains this in terms of four elements.
Symptom: What appears (to the main character) to be the problem that needs solving.
Response: What the main character does to address the symptom.
Problem: The real problem, which may be discovered or realized later in the story.
Solution: What would actually solve the real problem (which, in the case of a tragedy or tragi-comedy, never appears).